Fractals and fractal analysis are revolutionary mathematical concepts, touted by many as the universal geometry of nature. These new concepts are ideally suited to characterize the natural variability in physiologic form and function which have previously been described as random. They are providing new perspectives and fresh insights into pulmonary physiology. The proposed work Will use fractal analysis to further characterize the heterogeneity and spatial correlation of pulmonary blood flow distribution. Three interrelated approaches will be used. A new concept of spatial correlation, derived directly from fractal concepts, will provide a means of quantifying the degree of similarity in blood flow between spatially related regions of lung. Experiments will be performed which will quantify the relative effects of gravity, the structure of the vascular tree, and vasomotor tone on the regional heterogeneity of blood flow. The influence of these factors on the spatial correlation of blood flow will also be investigated. A three dimensional fractal model of pulmonary blood flow will be developed and will incorporate the knowledge obtained from the above studies to accurately depict the influences of the pulmonary vascular. tree and gravity on the distribution of blood. Compared to the currently accepted gravitational model, the fractal model will provide a more faithful representation of blood flow at the microcirculatory and gas exchange level.